In recent years, there has been a strong social demand for reducing the amount of a CO2 exhaust gas for resource saving, energy saving and environmental protection. In order to reduce the amount of a CO2 gas discharged from automobiles, various countermeasures such as weight reduction of them or use of electric energy are under investigation. As a common theme in automobiles, an improvement of fuel-cost-saving performance by improving the rolling resistance of a tire is regarded to be necessary. At the same time, automobiles are desired to have improved safety upon traveling. Such fuel-cost-saving performance and safety of automobiles are largely influenced by the performances of tires used therefor so that there is a strong demand for improving fuel-cost-saving performance, traveling stability and durability of automobile tires. Such properties of tires depend on various factors including structure and raw materials used for them. In particular, the performances of a rubber composition used for the tread part of a tire to be brought into contact with a road surface largely affects the properties of a tire such as fuel-cost-saving performance safety and durability. Under such situations, technical improvements in a tire rubber composition are now frequently studied and proposed and some have already been industrialized.
A tire tread is requested to have, for example, a small hysteresis loss for improving its fuel-cost-saving performance, have high wet skid resistance for improving its controlling stability and have excellent wear resistance for improving its durability. However, a reduction in hysteresis loss and high wet skid resistance are not attained simultaneously, which also applies to the relationship between wear resistance and wet skid resistance. It is difficult to satisfy all the above-described demands for an automobile tire by improving only one performance and an improvement of their balance is important. Typical methods for improving a tire rubber composition is to improve raw materials to be employed. An improvement in the polymer structure of a raw material rubber such as SBR or BR or improvement in the structure or composition of a reinforcing filler such as carbon black or silica, vulcanizing agent or plasticizer has now been carried out.
One of the most attractive techniques among them in recent years is to use silica as a reinforcing filler instead of conventionally employed carbon black. The typical technique of it is proposed, for example, in U.S. Pat. No. 5,227,425 wherein the balance of a tread rubber composition between a fuel-cost-saving performance and wet skid resistance is improved by adding, to SBR of a specific structure, silica as a reinforcing filler and kneading the mixture under specified conditions. A rubber composition using silica as a reinforcing filler however involves some problems to be solved. For example, since silica has low affinity with a rubber compared with the conventionally employed carbon black, its dispersibility in the rubber is not always good and this inferior dispersibility tends to cause insufficient wear resistance and insufficient strength properties. It is therefore necessary to improve the dispersibility of silica by kneading under particular temperature conditions in the presence of a silane coupling agent typified by bis-(triethoxysilylpropyl)-tetrasulfide and in addition, increasing a kneading frequency.
Under such situations, a method of modifying the end of a rubber with an alkoxysilyl group and silica-containing rubber compositions prepared thereby are proposed in JP-B-62-227908, JP-B-8-53513, JP-B-8-53576 and JP-B-9-225324 with a view to improving the dispersibility of silica in the rubber and reducing the amount of the silane coupling agent. The polymer modified by an alkoxysilyl group is available by reacting an active end polymer, which has been obtained by anionic polymerization, with a specific alkoxysilane compound. Such a polymer is however accompanied with the problems that the alkoxysilyl group of the resulting polymer tends to be condensed by water content, thereby causing a change in the viscosity of the polymer with the passage of time and in spite of an improvement in the dispersibility of silica, the resulting rubber composition has not always good processability owing to an increase in its viscosity.
A silica-containing composition using an epoxydated polymer is proposed in JP-B-9-118785 and JP-B-9-221429. It is however accompanied with the problems that it needs a special epoxidation step with hydrogen peroxide or peracid upon obtaining a modified polymer and in addition, it has not always good processability.
In JP-B-7-330959, proposed is a tire tread composition using SBR of a special structure, which has been obtained by coupling with a diglycidylamino-containing polyfunctional compound, with a view to improving processability in production steps, reducing rolling resistance and improving wet skid resistance. In this case, incorporation of carbon black in at least a predetermined amount is required in order to impart the composition with performances such as processability and wear resistance and to suppress radio frequency noise. It is further disclosed that the molecular weight distribution of the polymer falls within a specific range, the styrene content and 1,2-bond content, each within a specific range are preferred, and at least one unreacted glycidyl group is preferably contained in the molecule.
An object of the present invention is to provide, in consideration of such situations, a silica-containing diene rubber composition which has excellent processability even if a content of carbon black is small and is improved in both the balance between low rolling resistance and wet skid resistance and strength properties.